Although there have been extensive studies of rodent breast tumor models, and of human breast cancer cell lines in culture, there is still need for a human tumor model which can be manipulated experimentally but also provides a valid expression of the tumor cells in a host environment. Athymic nude mice bearing transplanted human breast tumors have been proposed as such a model. This review therefore discusses the use of the athymic nude mouse model of the study of human breast cancer biology, and focuses on four subjects: 1. biological characteristics of heterotransplanted breast tumors; 2. endocrinology and pharmacology of hormonal agents in the nude mouse; 3. endocrine sensitivity of heterotransplanted tumors; and 4. applicability and limitations of this model for the study of human breast cancer.